Since current and virtually all further telephone networks are and will be digital as opposed to analog, so-called TV phone service will enjoy wide spread use. TV phone service adds picture information to voice information which are communicated using a telephone network.
A typical signal standard, Common Intermediate Format (CIF) used in encoding a video signal for TV phone, different from a conventional video signal standard (NTSC system or the like), is described in the Journal of Institute of Television Engineers of Japan, Vol 46, No. 4, 1992, pp. 117-120 (Reference 1). When, for example, the CIF signal standard is used to encode a video signal for a TV phone, an image pickup element and an image pickup apparatus specifically designed to accommodate the CIF standard are required so as to constitute a TV phone. However, such an image pickup element is not manufactured at present, thereby requiring a video signal (analog signal) obtained from a currently available imager designed to accommodate, for example the NTSC signal standard to be converted into digital by processing of an analog/digital (A/D) converter, and converted into a sampling frequency to fit the CIF standard.
A typical embodiment of an imager is illustrated in FIG. 3. The imager 102 illustrated in FIG. 3 includes a plurality of photo diodes from which converted signal charges are transferred through vertical charge coupled devices (CCD)s to horizontal CCDs in synchronism with a horizontal scanning pulse. When the .imager illustrated in FIG. 3, is used as in most applications as an NTSC imager the number of photo diodes therein is set to a particular value so as to provide the appropriate number of samples for the image. The number of photo diodes to be included in the imager is called the effective pixel number. For an NTSC imager the effective pixel number is 510 horizontal pixels and. 492 vertical pixels. In order to read out all of the samples for each frame to be displayed at a normal aspect ratio, the horizontal scanning pulse is set to a particular frequency f.sub.0. For the NTSC Signal standard the frequency f.sub.0 of the horizontal scanning pulse is 9.6 MHz.
Aspect ratio is concerned with the dimensionally accurate display of an .image using the complete screen relative to the dimensions of the actual object. For example, the image of an object shown in FIG. 8A is displayed at a normal aspect ratio in FIG. 8B as opposed to that shown in FIG. 8C. FIG. 8C illustrates an image being displayed at an aspect ratio wherein the video signal thereof is contracted in the horizontal direction.
Thus when the imager illustrated in FIG. 3 is operated as an NTSC imager the vertical and horizontal synchronous (SYNC) pulses shown in FIGS. 7A and 7B causes the generation of the horizontal scanning pulse shown in FIG. 7C as HCCD CLK which in turn causes the information to be output from the imager in the manner shown as CCD OUT in FIG. 7D. The imager operated in such a manner produces a video signal such as that shown in FIG. 4.
The CIF standard operates at a frequency of 6.7 MHz which is different from the video signal of a frequency of 9.6 MHz produced by currently available pickup elements. Thus, some type of conversion circuit is necessary in order to interface the video signal produced by currently available imager and telephone networks operating in accordance with the CIF signal standard.
Further, since there is a need for digitally recording the video signal from the imager a digital apparatus record is required. Development of a digital video tape recorder (VTR) for such an application has been difficult due to differences in the required signal standard of a digital VTR as described in the Journal of Institute of Television Engineers of Japan, Vol. 45, No. 2, 1991, pp. 67-70 (Reference 2), the CIF signal standard and the frequency of the video signal produced by currently available imager due to their construction. An image pickup apparatus suitable for a digital VTR for use with a telephone network has not yet been developed.
As described above, in order for a video signal to be converted into a digital signal of a sampling frequency of the CIF standard for use in a TV phone system, a problem exists in that a conversion circuit of large scale and complexity must be used. A large scale complex circuit is also required to convert a video signal to a digital signal having the sampling frequency of the digital VTR standard.